Fix timezone handling in history endpoint and tests

[davisagli]

Fixes #1617

Also fixes the StaticTime helper used in tests to specify that the fake times used in tests are UTC, which should help with the problem where the times produced in tests are different depending on the local timezone of the machine where they run.

(This causes many example responses to be updated to add +00:00 to the serialized datetimes. This better matches the actual serialized datetimes that are produced when not running tests.)

[mister-roboto]

@davisagli thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

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[davisagli]

@jenkins-plone-org please run jobs

[jensens]

@jenkins-plone-org please run jobs

[davisagli]

@jenkins-plone-org please run jobs